"Nyo" is basically a replacement for the copula, "desu" and it's acceptable variants ("desu wa," "de gozaimasu," "de gozaru," and so on), when it's the last word of a sentence.
Even though there are so many different forms of "desu," differing in politeness, historical use, and dialect, it's fairly offensive to invent new ones. It's not exactly insulting, but it shows disrespect for the Japanese language and expresses unnecessary individuality (tradtionally not the best thing to do in Japan, though the times they are are a-changin').
The word is also related to "nyan," (meow). Appropriate, since Dejiko (no relation) is a catgirl.
Dakara, "nyo" tte ja nai pyo. "Nyo" wa orudo-sukuuru pyo ^^